1. Field of the Invention
This invention relates to silicon carbide coated carbon composite material and to a method for making the material.
More specifically, the present invention relates to a silicon carbide coated carbon matrix/carbon fiber (C/C) composite material which can be used, for example, as a susceptor (wafer holder) in the epitaxial chemical vapor deposition process in manufacturing semiconductors, as a heater or crucible in a semiconductor crystal pulling process, as a wall material in a nuclear fusion furnace, as a turbine blade, as a brake pad material for aircraft, and as a heat resistant material, e.g., an exterior wall of an aircraft or a spacecraft. The material of this invention is suitable for use as a component which is required to have high heat resistance and reliability (long life).
2. Description of Related Art
Carbon composite material which comprises carbon matrix and carbon fiber is recently finding many uses because of its high strength. Normally, such a carbon composite material comprises carbon matrix and carbon fibers for reinforcement. Threads, whiskers, textile, and nonwoven textile, which comprise carbon (graphite), are used as fibers. Those fibers are distributed in the carbon matrix in a parallel direction, two dimensionally or three dimensionally. All such carbon matrix/carbon fiber composite materials are referred to as carbon composite material in this specification.
Conventionally, silicon carbide coated carbon composite material is used for chemical or mechanical applications because of its excellent chemical resistance and mechanical strength, for example, in the semiconductor industry. In these conventional silicon carbide coated carbon composite materials, the silicon carbide coating is formed by one or more of the methods described below, individually or in combination:
a) Chemical vapor deposition (CVD) method. This is the most popular method. In this method, silicon carbide coating is formed from chemical reaction of gaseous materials. Relatively high density, tight coatings can be accomplished.
b) Siliconizing method. In this method, a silicon containing gaseous material is introduced at the surface of the carbon composite material, and a certain range of the carbon from the surface of the carbon composite material is siliconized to become silicon carbide.
c) Spreading method. In this method, a material comprising silicon carbide is painted on the carbon composite material and subjected to heat to form a coating.
d) Impregnation method. In this method, carbon composite material is prepared to have a relatively porous surface, and melted silicon liquid is introduced at the surface of the carbon composite. Carbon in a certain range from the surface of the carbon composite material is siliconized by a reaction with melted silicon. The coating produced by this method comprises silicon carbide and metallic silicon.
Conventional silicon carbide coated carbon composite materials are usually used under severe thermal conditions in which the material receives a thermal shock, such as rapidly heating and rapidly cooling, e.g., from room temperature to 1200.degree. C. or the reverse. When the silicon carbide coated carbon composite material is used under such conditions for a long time, microcracks are produced in the coating due to the difference of the heat expansion coefficient between the silicon carbide coating and the carbon composite material. In some cases, because of the extension of the microcracks, the coating will peel off from the surface, or oxidation of the carbon composite material will occur.
An attempt to absorb and relieve stress caused by the difference of the thermal expansion coefficient between the two materials has been carried out. In this attempt, with reference to FIG. 3, a siliconized layer 12 is formed in the carbon composite material near the surface before forming a silicon carbide coating 13. The carbon fibers in the carbon composite material are also siliconized near the surface to a depth of about 30 .mu.m to about 200 .mu.m or more, and this siliconization of carbon fibers worsens the properties of the surface area because of the loss of carbon fibers. This means there are no differences from normal carbon material, i.e., the surface is no longer carbon fiber reinforced carbon material. In such a case, the difference of thermal expansion coefficient is not-improved. It also decreases not only the thermal shock resistance but also the strength of the carbon composite material.
There is a need to provide a silicon carbide coated carbon material which has improved thermal shock resistance and high mechanical strength.
There is also a need to provide a method for producing such an improved silicon carbide coated carbon composite material.